A wireless device (e.g., a cellular phone or a smartphone) in a wireless communication system may transmit and receive data for two-way communication. The wireless device may include a transmitter for data transmission and a receiver for data reception. For data transmission, the transmitter may modulate a radio frequency (RF) carrier signal with data to generate a modulated RF signal, amplify the modulated RF signal to generate a transmit RF signal having the proper output power level, and transmit the transmit RF signal via an antenna to another device such as, for example, a base station. For data reception, the receiver may obtain a received RF signal via the antenna and may amplify and process the received RF signal to recover data sent by the other device.
The wireless device may transmit and receive communication data through a communication medium. In one example, the communication medium may be a wireless communication medium where communication data is transmitted and received by communication devices according to a wireless communication protocol. Example wireless communication protocols may include IEEE 802.11 protocols (e.g., Wi-Fi) and BLUETOOTH® protocols according to the Bluetooth Special Interest Group. Moreover, example wireless communication protocols may further include Long Term Evolution or LTE. LTE is a set of enhancements to the Universal Mobile Telecommunications System (UMTS) mobile standard promulgated by Third Generation Partnership Project (3GPP). In some examples, LTE provides over-the-air wireless communication of high-speed data for mobile phones and data terminals.
Analog signals within a wireless device may undergo amplification during various processing operations. Thus, amplifiers may be included within the wireless device to provide signal amplification. Different types of amplifiers may be available for different uses. For example, a wireless device such as a cellular phone may include a transmitter and a receiver for bi-directional communication. The transmitter may use a driver amplifier (DA) and a power amplifier (PA), the receiver may use a low noise amplifier (LNA), and the transmitter and receiver may both use variable gain amplifiers (VGAs).
Various classes of amplifiers may be used to implement the different types of amplifiers. A “class-D” amplifier, for example, may provide relatively power efficient operation by producing pulse width modulated (PWM) output signals. The PWM output signals may be generated by operating output transistors of the class-D amplifier as switches rather than operating them as linear gain devices. Operating the output transistors as switches may consume less power than operating the output transistors as linear gain devices.
Some class-D amplifiers may be differential class-D amplifiers designed to receive and amplify differential input signals and generate associated differential PWM output signals. Some differential class-D amplifiers may feedback a portion of the differential PWM output signals (e.g., the differential PWM output signals may be partially fed back) to be summed with the differential input signals. The feedback signal may control, at least in part, a frequency response associated with the differential class-D amplifier. The switched nature of the differential PWM output signals may cause an associated common-mode voltage to rise and/or fall uncontrollably. Relatively high common-mode voltages may stress one or more components within the differential class-D amplifier and may increase distortion associated with the differential input signal.
Thus, there is a need to control the common-mode voltage associated with the inputs of differential class-D amplifiers.